The presence of ambient acoustic noise in an environment can have a wide range of effects on human hearing. Some examples of ambient noise, such as engine noise in the cabin of a jet airliner can cause minor annoyance to a passenger. Other examples of ambient noise, such as a jackhammer on a construction site can cause permanent hearing loss.
Techniques for the reduction of ambient acoustic noise are an active area of research, providing benefits such as more pleasurable hearing experiences and avoidance of hearing losses.
In one of the simplest noise reduction techniques, an earcup can be designed such that its size, fit to a wearer's head, and sound absorption properties cause passive attenuation of ambient acoustic noise. For example, hearing protection ear muffs such as those worn on the flight deck of an aircraft carrier can be designed to absorb and reflect potentially damaging acoustic noise.
To further improve acoustic noise reduction, more sound absorbing material can be used, the size of the earcup can be increased, or the fit of the earcup to the wearer's head can be improved. However, there is a tradeoff between the bulkiness and comfort of hearing protection devices such as ear muffs and the amount of passive noise attenuation that they provide. To thoroughly reduce ambient noise, the ear muffs may need to be unreasonably large and/or uncomfortable. Instead, designers of such devices specify an acceptable amount of noise that is allowed to reach the wearer of the device.
Passive noise reduction is most effective at high frequencies (e.g., those frequencies that lie above 3 kHz) with reduced effectiveness below those frequencies. Furthermore, the effectiveness of passive noise reduction is susceptible to factors related to the coupling of the device onto the ear. Factors such as the shape of a user's head, the presence of glasses, etc. all affect the seal of the device around the ear, allowing additional noise to reach the wearer of the device.
Due to the shortcomings of passive noise reduction techniques, some designers of noise reduction systems use electronics to actively reduce noise. Referring to FIG. 1, an exemplary acoustic noise cancellation system 100 incorporates electronics that are designed to detect unwanted acoustic noise 104 that is not cancelled by passive attenuation provided by an earcup 101. The system 100 then uses a feed-back path to cancel the detected noise by creating an “anti-noise” signal (i.e., a signal that is equal and opposite to the detected noise). For example, a simple feed-back path 114 may be established by using a microphone 106 to sense unwanted acoustic noise in a cavity formed by a coupling of the earcup 101 and a wearer's head 109, and convert it to an electrical signal. The electrical signal is passed to a feed-back compensator 110 where it is amplified and phase inverted to generate the anti-noise signal. The anti-noise signal is then presented to the wearer's ear 108 using a transducer such as a headphone driver 112. Within the cavity, the transduced anti-noise signal and the unwanted acoustic noise 104 combine destructively, resulting in reduction of the net acoustic noise inside the earcup. This type of feed-back noise reduction is typically most effective at the low and middle audio frequency range (e.g., less than 1 kHz). It is difficult to increase this bandwidth due to limits placed on the acoustic system in terms of acoustic transport delay.
Feed-back active noise reduction systems such as the system presented in FIG. 1 typically exhibit a region of poor attenuation around 1 kHz (or in the “mid band”). As mentioned above, this is due to the passive attenuation being most effective at frequencies greater than 3 kHz and the feed-back attenuation being most effective at frequencies less than 1 kHz.
One solution for increasing noise attenuation around 1 kHz is a feed-forward filter spanning the aforementioned frequency band. Referring to FIG. 2, another exemplary acoustic noise cancellation system 200 includes an open loop feed-forward path 220 in addition to the previously presented feed-back path 114 to improve the attenuation of unwanted acoustic noise 104. The feed-forward path 220 senses the unwanted acoustic noise 104 in the environment outside of the earcup 101 using a second microphone 216 and converts it to an electrical signal. The feed-forward path 220 then processes the electrical signal using a fixed feed-forward compensator 218 which filters the electrical signal. The filter characteristic of the fixed feed-forward compensator 218 represents the typical passive attenuation provided by the earcup 101. The filtered electrical signal is used to create an anti-noise signal that is an estimate of the inverse of the noise that is not passively attenuated by the earcup 101. The anti-noise signal is presented to the wearer's ear 108 using a transducer such as the headphone driver 112. This method of feed-forward filtering can be more effective than the passive and feed-back attenuation in the 1 kHz region. at the frequency range that the passive and feed-back attenuation is ineffective (i.e., 1 kHz to 3 kHz).
Due to their open loop designs, the aforementioned systems are not capable of adapting to changes that occur in more dynamic environments. In particular, changes to the fit due to inconsistent coupling of the earcup 101 to the head of the earphone wearer 109 can degrade the noise attenuation performance of such systems.
Some adaptive noise cancellation systems actively compensate for dynamically changing aspects such as coupling. For example, a system may use an adaptive algorithm such as the LMS algorithm to continually adjust the coefficients of a feed-back and/or feed-forward filter based on a cost function derived from the amount of noise sensed near the wearer's ear. While such systems may be effective, they can require complex, power intensive hardware and significant processing time for measuring noise, then calculating and synthesizing appropriate anti-noise signals in real time. Furthermore, the speed of convergence of the LMS algorithm can be slow in the presence of non-stationary noise and at high frequencies. Thus, such a system may be impractical for small, low cost, low power applications such as consumer headphones and earphones.
There is a need for a simple, fast, and low power active noise reduction system that is capable of compensating for variations due to changes in coupling.